Why is italic style used in theorems and lemmas instead of slanted style?

Question

It seems unintuitive to me to use italic text in theorem and lemma text styles, because some mathematical symbols (letters) are almost identical to regular italicised letters.

Consider the following example (text sourced from this website). I got the wonderful newtheoremstyle solution from this SE thread, from a user named Mico. Thank you, Mico!

\documentclass{article}

\usepackage[utf8]{inputenc}
\usepackage[english]{babel}
\usepackage{amsthm}

\newtheoremstyle{bettertheorem}
{}                % Space above
{}                % Space below
{\slshape}        % Theorem body font % (default is "\upshape")
{}                % Indent amount
{\bfseries}       % Theorem head font % (default is \mdseries)
{.}               % Punctuation after theorem head % default: no punctuation
{ }               % Space after theorem head
{}                % Theorem head spec

\begin{document}

\newtheorem{theorem}{Theorem}

\begin{theorem}
Given two line segments whose lengths are $a$ and $b$ respectively there is a 
real number $r$ such that $b=ra$.
\end{theorem}

\theoremstyle{bettertheorem}
\newtheorem{bettertheorem}{Theorem}

\begin{bettertheorem}
Given two line segments whose lengths are $a$ and $b$ respectively there is a 
real number $r$ such that $b=ra$.
\end{bettertheorem}

\centering
\huge{\textbf{Which one looks better to \emph{you}?}}

\end{document}

Now, I can see how this question could get flagged as "primarily opinion-based", so let me rephrase my question as objectively as possible: Is there any rational reason to use italic style instead of slanted in theorem texts, considering that italicised text and math text look more alike (and are, at least for me, harder to distinguish and read) than slanted text and math text?

Thank you for your answers in advance and God bless.